1. Field of the Invention
The invention relates to a method and apparatus for use in drilling a well from an offshore floating vessel, the vessel having a marine riser interconnected between the vessel and a subsea wellhead assembly.
2. Description of the Prior Art
An increasing amount of offshore deepwater exploratory well drilling is being conducted in an attempt to locate oil and gas reservoirs. These exploratory wells are generally drilled from floating vessels. As in any rotary drilling operation, drilling fluid must be circulated through the drill bit in order to cool the bit and to carry away the cuttings. This drilling fluid is normally returned to the floating vessel by means of a large diameter pipe, known as a riser, which extends between a subsea wellhead assembly and the floating vessel. The lower end of this riser is connected to the wellhead assembly which is generally adjacent the ocean floor, and the upper end usually extends through a centrally located hull opening of the floating vessel. A drillstring extends downward through the riser into earth formations lying below the body of water, and drilling fluids circulate downwardly through the drillstring, out through the drilling bit, and then upwardly through the annular space between the drillstring and riser, returning to the vessel.
As the water depths for these drilling operations continue to increase, the length of the riser and subsequently its unsupported weight also increases. The riser has the same structural buckling characteristics as a vertical column, such that riser structural failure may result if compressive stresses occur over a long length of the riser. To avoid the possibility of this occurrence, riser tensioning systems are installed on board the vessel, which apply an upward force to the upper end of the riser, usually by means of cable and sheave mechanisms connected between the vessel and the upper elements of the riser.
Since the drillstring rotates as it extends downward through the riser, any inclination of the riser away from a vertical plane defined from the drilling equipment located on the vessel to the wellhead assembly will cause the drillstring to contact the interior surface of the riser. Continuous drillstring rotation will cause chafing at the points of drillstring and riser contact. Any unnecessary bends or "dog-legs" in the riser therefore need to be avoided.
Where the riser's lower end connects to the wellhead assembly, the riser must be aligned vertically, to prevent the rotating drillstring from chafing and thereby damaging the blowout prevention equipment located immediately above the wellhead assembly. The well blowout prevention equipment must be maintained in an operable condition at all times.
To align the riser vertically at the wellhead assembly, when the riser is subjected to strong ocean currents, the vessel must be offset from, or moved away from, a location directly above the wellhead assembly. This offset position is necessary to compensate for the slight bend that develops in the entire riser assembly due to the ocean currents. For example, a riser 6000' in length may require a vessel offset of 100 to 400 feet.
Due to this vessel offset, the upper elements of the riser tend to develop an angle of inclination from the vertical. But the upper end of the riser is typically secured vertically to the vessel, to allow vertical insertion and removal of the drillstring. The inclined position of the riser directly below the riser's vertical oriented vessel attachment point is permitted by the installation of a flexible coupling called a ball joint or flex joint, beneath the vessel attachment point. With the vessel offset 200 to 300 feet from the wellhead assembly in a water depth of 5000' to 6000', the inclination of the upper end of the riser may easily be 4.degree. or more, causing this amount of misalignment between the inserted drilling equipment and the upper end of the riser.
But if the drillstring continuously rotates within this flexible coupling while the coupling is misaligned 4.degree. end-to-end, chafing and subsequent damage will result, with possible separation of the drillstring and/or the riser. Any separation of either the drillstring or the riser will require expensive retrieval and repair operations with subsequent delays in field development.
An apparatus and method need be developed that reduces the angle of misalignment of the riser's upper section with the vessel's drilling equipment, to prevent failure of either the riser or the drillstring.